The increase in air traffic in the last 50 years and the growth assumptions for the next 20 years are leading to a progressive saturation of the frequency band allocated for aviation voice communications. The solution found by those involved in the aviation world is a progressive migration from “voice” as a means of communication to the digital link (“data”). This involves converting into a set of given messages the “voice” instructions and dialogues between the ground and the cockpit. The flight settings thus converted are called flight clearances. The clearances are sent from an air traffic control (ATC) station on the ground by means of a digital link to a flight management system (FMS) of an aircraft. The clearances are communicated by the ATC to a ground/cockpit communication system called the CMU (“Communication Management Unit”) of the FMS. During the flight, unexpected events occur that will modify the flight plan that the aircraft is in the process of following, called the active flight plan. On this occasion, the ATC sends to the aircraft, by means of digital links, flight clearances that can be classified into several categories:                communication identification and management messages CONTACT BRELO 123.00, DUE TO TRAFFIC etc. . . . , intended to be displayed on a screen and to be sent to an item of frequency-selection equipment,        context and response messages of the type UNABLE, STANDBY, ROGER, etc. . . . , intended to be displayed on a screen,        control clearances which correspond to actions to be performed, of the type “CLIMB”, “CROSS”, “DEVIATE”, “REDUCE SPEED”, “MAINTAIN[speed]” etc. The actions are intended to modify the active flight plan.        
The navigation-aid system FMS conventionally assists the crew in the programming of the flight plan before take-off and in following the trajectory of the flight plan from take-off to landing. A flight plan is a trajectory assumed to have to be followed by the aircraft. The trajectory comprises a lateral trajectory which is usually characterized by a chronological sequence of segments connecting, by twos, waypoints described by their position in the horizontal plane and arcs of a circle both for making the heading transitions between segments at the waypoints and for following certain curved segments. The trajectory also comprises a vertical profile, corresponding to an estimate of the trajectory of the aircraft in the vertical plane, optimized on the lateral trajectory. The waypoints are characterized by their respective passage times.
Conventionally, the FMS comprises an FPLN (“Flight Planning”) function for managing the flight plans which computes a flight plan. This function conventionally manages a plurality of memory spaces capable of storing various types of flight plan including:                an active flight plan: which is the flight plan currently being carried out by the crew, on which the aeroplane is guided,        a temporary flight plan, or modified flight plan: which is a working flight plan, conventionally created from an active flight plan in order to modify the active flight plan. More precisely, the temporary flight plan is created from an active flight plan in order to take account of instructions, modifying the active flight plan, which will be effective within a time horizon of less than a few minutes (of less than one minute to five minutes, limits not included, depending on the necessary speed of reaction corresponding to the flight phase that is in progress). The data stored in the memory space dedicated to the active flight plan are copied into a memory space dedicated to a temporary flight plan. This is a temporary draft which allows the crew to make changes to the active flight plan and to view the temporary flight plan thus created before activating it. When a temporary flight plan is activated, this has the effect of replacing the active flight plan with the temporary flight plan and of deleting the temporary plan. More precisely, the content of the memory space dedicated to the active flight plan is replaced by the content of the memory space dedicated to the temporary flight plan.        one (or more) secondary flight plan(s), which are additional flight plans usually used to make more strategic modifications, that is to say which are taken into account in the longer term. The secondary flight plans are used to take account of the instructions, modifying the active flight plan or relating to a new flight plan, which will take effect on a more distant time horizon than for the temporary flight plan (typically at least equal to 5 minutes). The secondary flight plan can be used for predicted break routes, in order to prepare the next flight in domestic operations or to consult the impact of a route change. Secondary flight plans are not necessarily associated with the aeroplane in which they are produced. The pilot may enter therein, for example, his return flight plan, or flight plans with different diversion and/or destination-change options. The secondary flight plan can be activated at any time. In this case, the data stored in the dedicated space are not necessarily deleted in order to allow the pilot to test changes to the flight plan which has become active. The secondary flight plan can also be created from the active flight plan.        
Secondary and temporary flight plans differ in their presentation to the crew (different pages, different colours, different change options, different prediction computation assumptions). These types of flight plan are specified in the international aviation standard AEEC ARINC 702A “Flight Management System”, in section “4.3.2—Flight planning”. The temporary flight plan respectively secondary flight plan is identified as the “modified flight plan”, respectively the “secondary flight plan.”
Currently, manufacturers include the control clearances in a default flight plan. In other words, a default flight plan is computed based on the control setting. The default flight plan is, for certain manufacturers, the active flight plan, for others the temporary flight plan and for some a secondary flight plan. Because the default flight plan is always the same flight plan, the inclusion of a control setting in this flight plan is not always suitable for the use that the pilot makes of it as a function of the flight setting. The pilot is therefore regularly required to transfer a clearance into a flight plan other than that in which it has been included. This type of action is time-consuming for the pilot.
The object of the invention is to alleviate the aforementioned drawbacks.